Aaron Alexis, who the FBI believes to be responsible for the shootings at the Washington Navy Yard in Washington, DC (Reuters)

US-based security services provider USIS that did background checks on former NSA contractor Edward Snowden has admitted that it had also cleared Aaron Alexis, the Washington Navy Yard shooter, in 2007.

The Virginia-based company, which is one of the largest security services providers in the US, had earlier denied carrying out background checks on Alexis, according to Bloomberg.

"Today we were informed that in 2007, USIS conducted a background check of Aaron Alexis" for the US Office of Personnel Management (OPM), a USIS spokesman said in an e-mail to the news agency.

Alexis killed 12 people at the Navy Yard on 16 September and was then shot dead by the police. He had worked for a Florida-based IT consulting firm and received a secret-level clearance from the contractor to access military facilities such as the Navy Yard.

"We are contractually prohibited from retaining case information gathered as part of the background checks we conduct for OPM and therefore are unable to comment further on the nature or scope of this or any other background check," the email added.

Alexis had been arrested three times for suspected offences involving guns, but was not charged or convicted.

While USIS admits that it had conducted Alexis's background check, there was no evidence that the company did anything wrong in the vetting.

The OPM has "reviewed the 2007 background investigation file for Aaron Alexis, and the agency believes that the file was complete and in compliance with all investigative standards," the federal agency said in a statement.

Edward Snowden Case

USIS, which handles about 45% of all background checks for the OPM, is already under criminal investigation over its handling of background check of National Security Agency (NSA) whistleblower Edward Snowden. However, the company defended itself saying it met all standards in the procedure. It added that the NSA was ultimately responsible for approving Snowden's clearance.

"Under contract to the US Office of Personnel Management (OPM), the company performed a five-year "periodic reinvestigation for Mr Snowden's clearance in February 2011. USIS did not perform the background investigation supporting Snowden's initial security clearance," the company had earlier said in a statement.

Call to Overhaul Vetting Procedures

Following the Snowden incident, the Office of the Director of National Intelligence is currently reviewing the vetting procedures for contractors.

The White House had also announced a separate review by the Office of Management and Budget to examine "standards for contractors and employees across federal agencies".

Meanwhile, US lawmakers are calling to fix the issues in the government's background verification system.

"From Edward Snowden to Aaron Alexis, what's emerging is a pattern of failure on the part of this company, and a failure of this entire system, that risks nothing less than our national security and the lives of Americans," Bloomberg quoted Claire McCaskill, US Senator for Missouri, as saying.